1. Field of the Invention
The present invention is directed to a method as well as to an apparatus for controlling the pulse sequence in a nuclear magnetic resonance tomography system.
2. Description of the Prior Art
For the measuring sequence in a nuclear magnetic resonance tomography system, it is essentially the time curve of gradient currents, radio-frequency transmission pulses and sampling periods for the acquired MR signal that are to be controlled. In currently standard control systems, the control data sets are pre-calculated before the sequence starts. Stored time curves are thereby employed, for example for the leading and trailing edges of gradient currents. The measuring procedure is then xe2x80x9cplayedxe2x80x9d in the form of a list during the sequence execution. Such a controller is disclosed, for example, by U.S. Pat. No. 5,349,296.
Since the entire sequence execution is already determined at the start of the sequence, the further sequence execution can be influenced only with difficulty and in a very limited way after the sequence starts. Essentially, only a built-in stopping and waiting for an external trigger signal is possible. A large data volume must be stored. In order to keep this data volume somewhat within limits, parts of the sequence topology are typically imaged in the form of a loop structure. For example, loop instructions are contained in the dataset. The description xe2x80x9clanguagexe2x80x9d for the dataset thus becomes complex and inflexible.
U.S. Pat. No. 5,144,242 discloses a control arrangement for an MR apparatus wherein a memory for control commands is continuously reloaded from a bulk storage while the sequence is being processed. The memory for control commands is thus relieved. A similar control arrangement is also disclosed in U.S. Pat. No. 5,606,258.
German OS 44 35 183 discloses a method for the operation of a magnetic resonance apparatus wherein time-variable magnetic gradient fields are generated dependent on time-variant signals. The time-variant signals are represented by discrete signal values that are generated by a virtual machine by processing a virtual machine program. The processing is based on a prescription of parts of time curves for gradient currents. To that end, normalized time sub-curves (curve portions) are stored as a gradient table in a memory area of the virtual machine. These sub-curves, multiplied by corresponding scaling factors, are reshaped into the required signal values and are compiled to form the required gradient current signal curves. As a result, as in every sequence, occupation of k-space with magnetic resonance image data takes place during a radio-frequency reception phase in combination with a corresponding drive of a radio-frequency system, whereby the sequence program has available the normalized parts of the time curves for gradient currents in the way described above.
U.S. Pat. No. 5,512,825 discloses a method for minimizing dead times for the gradient start and end values as well as to specify a moment for the gradient curve.
In the definition of gradients, a distinction must be made between what are referred to as xe2x80x9clogicalxe2x80x9d gradients and the xe2x80x9cphysicalxe2x80x9d gradients. This distinction is necessary because that arbitrarily oblique slices can be imaged with nuclear magnetic resonance tomography apparatus. These oblique layers are defined by prescribing logical gradients that are oriented correspondingly oblique in a Cartesian coordinate system. Each gradient coil, of course, can supply only one gradient in a defined direction, namely only in one axis of a physical Cartesian coordinate system. These gradients are referred to as physical gradients. The oblique logical gradients are therefore generated by superposition of physical gradients, by a vector addition as considered mathematically. The following problem, however, arises. In conventional systems, the logical gradients are typically defined in the form of trapezoidal pulses. Given a superposition of a number of physical gradients, the physical gradients then typically are no longer trapezoidal, but exhibit a complex current-time curve that must be governed by the gradient amplifier. The maximum rise of the gradient edges of the physical is limited by what is referred to as the slew rate in any gradient system. Further, of course, the gradient amplitude is also limited. What rise times and what amplitudes occur in the physical gradients is not predetermined in the definition of the logical gradients since these are calculated from the logical gradients in the sequence execution. On the other hand, the limitation of the rise time and of the amplitudes of the physical gradients must be taken into consideration in the definition of prepared data sets for the logical gradients. For example, the worst case must always be assumed, i.e. that all gradient rises encounter a physical gradient. Only a poor utilization of the gradient hardware is thus possible.
An object of the present invention is to provide a method and apparatus for the control of a pulse sequence wherein the aforementioned disadvantages are avoided.
This object is inventively achieved in a method and an apparatus wherein a control dataset for gradients (i.e. for the gradient amplifiers) and for radio-frequency pulses and for nuclear magnetic resonance signal sampling (i.e. for the radio-frequency transmission and reception channel) is calculated from sequence data prescribed in the k-space during the running time of the pulse sequence. An intervention into the running sequence, i.e. a modification of the sequence parameters, is thus possible. A low memory capacity requirement is achieved due to a short xe2x80x9cdwellxe2x80x9d time of a maximum of one time slice during which data must be stored.